High-Speed Video Reveals Cats' Secret Tongue Skills

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High-Speed Video Reveals Cats' Secret Tongue Skills

High-speed videos reveal the strange technique and delicate balance of physical forces cats use to lap milk from a bowl.
Unlike dogs, who use their tongues as ladles to scoop water into their jaws, cats pull columns of liquid up to their mouths using only the very tips of their tongues.
"Cats are just smarter than dogs from the point of view of fluid mechanics," said civil engineer Roman Stocker of MIT.
Cats' tongues operate more like octopus tentacles or elephant trunks, Stocker found. The study's results could have implications for designing soft, flexible robots.
Stocker grew curious about how cats lap about three years ago while watching his housecat, Cutta Cutta, eating breakfast.
"I started to think, there had to be something interesting about the mechanisms of how the cat was getting the water or milk into the mouth, because it had to overcome gravity," he said.
Cutta Cutta's tongue flicked in and out of his mouth too quickly for Stocker to study with his naked eye. So he borrowed a high-speed video camera from a colleague at MIT.
Stocker and his colleagues filmed Cutta Cutta drinking a bowl of water mixed with a little bit of yogurt ("for visual contrast and palatability") at 120 frames per second. The results appeared Nov. 11 in Science.
Video: Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff and Roman Stocker

The videos showed that Cutta Cutta's tongue curled backward like a J as it approached the bowl.
Many scientists assumed the nubs that roughen cats' tongues help in drinking, perhaps by sticking to the water. But the videos showed only the smooth, upper tip of the tongue touched the liquid.
The cat's tongue moved as quickly as 2.5 feet per second, "which is quite incredible," Stocker said.
He lapped between 3 and 4 times a second, and drank about 0.14 milliliters of water per lap.
As Cutta Cutta's tongue returned to his mouth, a column of water was pulled along with it. The column was created by a balance between two forces: gravity, which pulls the liquid back toward the Earth, and inertia, the tendency of a moving object to keep moving unless something stops it.
Initially, inertia is more powerful than gravity, and the column rises with the cat's tongue. But eventually gravity starts to take over, and the column begins to fall back down toward the bowl.
The cat closes his mouth and pinches off the top of the column at just the right second to get the most liquid possible.
"There's a time when the volume of the column is largest, when inertia and gravity just balance," Stocker said. "That's when the cat laps."
Video: Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff and Roman Stocker

To make sure Cutta Cutta wasn't endowed with an especially talented tongue, the researchers filmed 9 other cats -- Shady, Gus, Blackie, Cosmic, Brazil, Loretta, Blackie 2, Bella, and one whose name is unknown -- at the Massachusetts Society for the Prevention of Cruelty to Animals. The videos showed similar results.
But studying actual cats, which are notoriously difficult to direct, was limiting, Stocker says.
"You can't force a cat to lap at a different speed or from different height, or change their tongue size for that matter," he said. "These are all aspects that are important to understand how the lapping mechanism works."
So the team built a model cat tongue in the lab, using a glass disk to adhere to the water and a special device to pull the disk up very quickly.
It was surprisingly difficult to find a machine that could match the speed of a cat's tongue, Stocker said. He ended up borrowing a device called a Filament Stretching Rheometer, which uses magnets instead of friction-prone mechanical gears to lift things quickly, from a mechanical engineer at MIT. There are only a few of these devices on the planet, Stocker says, and one on the International Space Station.
Stocker and colleagues filmed the robotic "tongue" at 1,000 frames per second and studied how quickly the volume of the water column changed. The videos confirmed their suspicions: Cats close their mouths at the exact best moment to get the most water.
Video: Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff and Roman Stocker

Stocker and colleagues then scaled their experiment up to great cats. Based on the earlier results, the researchers expected cats' lapping speeds to tail off with the cats' mass.
"The larger the animal, the slower it should lap," Stocker said.
The team took their high-speed camera to the Zoo New England and filmed a tiger named Luther, a lion (Christopher), an ocelot (Kuma) and a jaguar (Kanga) as they lapped.
They also slowed down YouTube videos of great cats, like the cheetah above.
"This is the first paper in Science that used YouTube data, as far as we understand," Stocker said.
Cats of all sizes seem to know exactly when the column is about to succumb to gravity, and they pinch it off right then.
"That's what surprised us the most," Stocker said. "The cat seemed to know exactly when to lap, and the larger felines did too."
Video: YouTube/netwarrior666

The fact that cats bend their tongues backward when they drink was first demonstrated in a 1940 film by Harold "Doc" Edgerton, also of MIT, who is famous for his high-speed videos of things like bullets piercing apples.
The 10-minute film, "Quicker 'n a Wink," won an Oscar in 1941.
This paper marks the first time that the fluid dynamics of how cats lap has been studied in detail, Stocker says.
"Biologists have been interested in ingestions of fluids, but most of the focus is on what happens when the fluid is already in the mouth, how does it get into the stomach and so on," he said. "The process of lapping has not been studied before."
Video: A leopard drinking. Credit: Africam.com

Marine biologist Mark Denny of Stanford thinks the findings are fun and surprising.
"The guys at MIT are really good at doing these really fun experiments," he said. "So much of science now is concerned with how we're going to take care of global warming or how we're going to fix cancer. It's so nice to see people doing science more or less for the fun of it."
But the findings could have implications beyond the realm of pure, fun science. The cats' tongues resemble flexible, muscular structures in several other creatures, saysWilliam Kier, a biomechanics expert at the University of North Carolina.
"The tongue is elongating using the same mechanisms that octopus arms use," he said. "This finding emphasizes the diversity and complexity and speed of movements that are possible using these hydrostatic mechanisms."
Kier's research has helped other scientists, including robotics engineer Ian Walker of Clemson University in South Carolina, design soft, flexible robots inspired by octopus tentacles and elephant trunks (above). The insights gleaned from cats' tongues could conceivably help design similar robots in the future.
"Trunks, tongues and tentacles have the ability to produce a bend at any point along the length of the structure," Kier said. Robotic arms based on the same principles "have the flexibility to handle objects that vary in mass or texture."
Video: The OCTARM robot. Ian Walker